1. Field of the Invention
The present invention relates to a compressor used in a refrigerating machine, and more particularly to a rotary compressor having a rolling piston and a vane that is moved with the rolling piston.
2. Description of Related Art
Chlorofluorocarbon such as the Freon R12 and R22 have been popularly used as a refrigerant in prior arts. Specifically, R12 has been widely used for a long time as an ideal refrigerant, as it is chemically stable, nonflammable, and nonpoisonous.
It has been realized, however, that R12 contains chloric atoms in its molecules which cause destruction of an ozone layer, thus it has been desired to develop and use a substitutional refrigerant.
Hydrofluorocarbon (HFC) which contains no chlorine is considered to be a practical substitute ("Hydraulic and Pneumatic Technology", June 1994, Japan Industrial Publishing Co.).
Without chlorine, HFC has less smoothness unlike R12 or R22, thus an ice machine oil to be used therewith is required to have high compatibility with HFC, so as to cause the refrigerant to be fluently flown to every part of the compressor, as well as to keep the efficiency of a heat exchanger. Mineral oil or alkyl benzene which has been conventionally used with Freon has extremely low compatibility with the substitutional refrigerant mentioned above, thus it is considered to use an ester oil which has high compatibility with the substitute ("Hydraulic and Pneumatic Technology", June 1994, Japan Industrial Publishing Co.).
However, the employment of such substitute refrigerant and ester oil causes abrasion of metallic materials such as cast iron (ex. FC25), carbon steel (ex. S-15C), cold forged steel (ex. SWRCH1OA), alloy steel (ex. SCM435), (all designated by Japanese Industrial Standard), sintered alloy steel, and stainless steel, used as a rolling piston and a vane which are slid with each other, resulting in a shorter life of the compressor. This is because ester oil is highly absorptive due to its polar group, in addition to the fact that HFC with no chlorine has a less lubricative effect. The absorbed water dissolves the ester oil to produce carboxylic acid, which causes corrosion on the surface of the metallic materials constituting sliding members such as a vane, shortening their fatigue lives ("Hydraulic and Pneumatic Technology", June 1994, Japan Industrial Publishing Co.). The dissolution of the ester oil also produces acid which intrudes into ferrous metals and causes stress corrosion, leading to a shortened life of the vane.
The sliding contact between the rolling piston and the vane tends to fall into boundary lubrication with an oil film partly broken because of the poor lubricating ability of the substitute refrigerant. The boundary lubrication creates cohesion between the contacting members when both materials are made of steel, and the abrasion is further accelerated to shorten their fatigue lives.
The rolling piston and the vane are thus desired to have long lives as they are incorporated into a compressor which is tightly sealed and operated for a long time without maintenance.
Japanese Published Unexamined Patent Application 5-084357 discloses a compressor for refrigerating machine having one sliding member of cast iron and the other sliding member of ferrous metal coated with a compound mainly composed of chromium nitride (CrN) formed by a physical vapor deposition (PVD) method.
Japanese Published Unexamined Patent Application 7-145787 discloses a compressor for a refrigerating machine comprising a vane made of ferrous alloy steel containing chromium or ferrous sintered steel, at least its tip portion coated with Chromium nitride ceramic after being nitrided to form a compound layer with iron, chromium, and nitrogen.
These coating films mainly composed of CrN disclosed in the above Applications help the sliding members to have high resistance against abrasion even with the substitute refrigerant without chlorine and its compatible ice machine oil. Nevertheless, an adequately long life cannot still be assured, as these coating films are soon scaled off, as realized according to the experiments repeatedly carried out. Referring now to FIGS. 1A and 1B, a coating film (b) at the tip of a vane (a) is first longitudinally cracked by the sliding movement with the rolling piston and the vane (a). Then, the crack (c) is broaden by an external force shown by an arrow (F) transversely applied to the edge of the crack (c) by the fractional force between the vane (a) and the rolling piston, and filly peeled off.
The vane (a) is usually finished grinding not to give a clearance in contact between the vane (a) and the rolling piston. Nonetheless, the surface of film (b) is laterally undulated when microscopically observed, each ridge extending longitudinally in a row at the tip of the vane (a), as shown in FIGS. 1A and 1B. This is because the coating film (b) is evenly formed by the accurate ion plating method along the minute unevenness on the ground surface of the vane (a). Such evenness on the ground surface of the vane (a) is formed by grinding the vane (a) with a longitudinal movement of a grindstone (j) having a radiusing groove (k) along the tip of the vane (a) as shown in FIGS. 2 and 3.
When the vane (a) is slid with the rolling piston, a raised portion (b1) of the coating film (b) is brought into linear contact with the rolling piston, and most tightly pressed between the rolling piston and a ridge (a1) formed on the ground surface of the tip of the vane (a). It is thus assumed that the scaling of the coating film is caused by the stress of pressed contact between the vane (a) and the rolling piston, making the longitudinal crack (c) on the hard coating material of CrN at the tip of the vane (a) along the ridge line of the raised portion (b1). This phenomenon is observed irrespective of whether the coating film (b) is formed only at the tip or on the entire surface of the vane (a).
It is also possible to form such coating film (b) by PVD methods. Though any of the PVD methods including vacuum evaporation, electric discharge plating, vapor plating, etc., is applicable, the ion plating method, including the reactive ion plating method and the high frequency ion plating method, is most operable and suitable to form a coating film (b) of good adhesion.
FIG. 4 shows a PVD apparatus employing the reactive ion plating method.
The pressure in a vacuum tank (d) is kept substantially at 10.sup.-3 Torr. Chromium is vaporized by an electronic gun (e) as a vapor source. An ion electrode (f) is biased with a positive voltage of about 50V for ionizing the vaporized chromium. The vaporized chromium is then beamed toward a base material (g) biased with a negative voltage, and collided thereagainst with a high kinematic energy. Nitrogen is used as a reactive gas, whereby a compound layer mainly composed of CrNx is formed on the surface of the base material (g).
However, when the vane (a) is processed as the base material to form a coating film (b) thereon by the ion plating method, electric charge is concentrated at the comers (a2, a2) of the vane (a), where the reaction and crystallization is progressed more actively than the other parts, resulting in raised parts (b2, b2) on the coating film though a micro level as shown in FIG. 5. As can be seen from FIG. 6, when the vane (a) is slid within a cylinder (i) with a micro clearance therebetween created by the raised parts (b2, b2), the cylinder (i) or the rolling piston (h) may be damaged by the raised parts (b2, b2), causing a leakage of the refrigerant, which leads to a shorter life of the product.